An error compensation technique for ensuring reliability of communication includes a forward error correction (FEC) scheme and an automatic repeat request (ARQ) scheme. In the FEC scheme, an error at a reception end is corrected by adding an extra error correction code to information bits. In the ARQ scheme, an error is corrected through data retransmission, for which a stop and wait (SAW) scheme, a go-back-N (GBN) scheme, a selective repeat (SR) scheme, and the like, are used. The SAW scheme is a scheme in which whether or not a transmitted frame has been properly received is first checked and a next frame is then transmitted. The GBN scheme is a scheme in which N number of successive frames is transmitted, and if the transmission is not successful, every transmitted frame following an error-generated (erroneous) frame is retransmitted. The SR scheme is a scheme in which only an error-generated frame is selectively retransmitted.
The FEC scheme is advantageous in that a time delay is small and there is no need to transmit and receive any extra information between a transmission end and a reception end, but a system efficiency is degraded in a good channel environment. The ARQ scheme has a high transmission reliability but is disadvantageous in that a time delay occurs and the system efficiency is degraded in a poor channel environment. Thus, in order to resolve such shortcomings, a hybrid automatic repeat request (HARQ) scheme combining the FEC and ARQ has been proposed. According to the HARQ scheme, whether or not data received by a physical layer has an error that cannot be decoded is checked, and if the data has an error, data retransmission is requested to thus enhance performance.
A mode of the HARQ may be divided into a chase combining mode and an incremental redundancy (IR) mode. The chase combining mode is obtaining a signal-to-noise ratio (SNR) gain by combining an error-detected data with retransmitted data, rather than discarding the error-detected data. The IR mode is incrementally transmitting additional redundant information in retransmitted data to thereby reduce a burden of retransmission and obtain a coding gain.
If no error is detected from the received data, a receiver transmits an acknowledgement (ACK) signal as a response signal to inform a transmitter about the successful reception. If an error is detected from the received data, the receiver transmits a negative-acknowledgement (NACK) signal as a response signal to inform the transmitter about the error detection. Upon receiving the NACK signal, the transmitter may retransmit data.
The receiver of the HARQ scheme basically attempts an error correction on received data and determines whether data retransmission should be made by using an error detection code. As the error detection code, a cyclic redundancy check (CRC) may be used. When the reception data is detected to have an error through the CRC detection process, the receiver transmits the NACK signal to the transmitter. Upon receiving the NACK signal, the transmitter transmits proper retransmission data according to the HARQ mode (Chase combining mode or IR mode). Upon receiving the retransmission data, the receiver combines the previous data and the retransmission data and decodes the same to thus improve reception performance.
The retransmission scheme of the HARQ may be classified into a synchronous scheme and an asynchronous scheme. In the synchronous HARQ, data is retransmitted at a point of time both the transmitter and the receiver knows about, and signaling required for transmission of data such as HARQ processor number or the like can be reduced. In the asynchronous HARQ, resources are allocated at an arbitrary time for retransmission. Because signaling is required for data transmission, overhead is generated.
The HARQ may be classified into an adaptive HARQ and a non-adaptive HARQ according to transmission attributes such as a resource allocation, modulation scheme, size of a transport block, and the like. The adaptive HARQ is a scheme in which the transmission attributes used for retransmission are compared with an initial transmission, and entirely or partially changed and transmitted according to a change in a channel state. The non-adaptive HARQ is a scheme in which the transmission attributes used for an initial transmission are continuously used regardless of a change in the channel state.
In the IR mode, retransmission data, which discriminately includes additional redundant information incrementally added to previously transmitted data, does not need to be transmitted with the same size as that of the initial transmission data like the non-adaptive HARQ. That is, transmitting the retransmission data with the same size as that of the initial transmission data is a waste of radio resources. And the use of the transmission attributes used for the initial transmission as it is each time cannot properly reflect a changed channel state when the retransmission data is transmitted.
In general, the need of retransmission means that a channel state is not good, and the retransmission data needs to be transmitted by using a modulation scheme with a good decoding performance. Thus, in the non-adaptive HARQ scheme in which the re-transmission data is transmitted in the IR mode, a method for enhancing efficiency of retransmission data is required.